Final Fantasy XIII - Summons
Only after defeating Summons or Eidolons, can you call them during battle.
When fighting Summons, you can't win by just attacking them. You'll need a
special strategy to be able to defeat them effectively. When a summon is
damaged by your attacks, a special gauge increases. When it is full, press
the square button to bring it under your control. Strategies can be found in
the walkthrough section.
After that, the character will learn the Summon Ability. Selecting this command
during battle (TP menu 3 points) will make the Summon appear, they will fight
along side of you until the Summon Point (SP) bar is empty, which will happen
when the summon is hit. It also decreases in time.